Seismic Swarm S20141118.1: Geological Context and Event Analysis in Greece

Greece lies at the convergent boundary between the African and Eurasian tectonic plates, where the Hellenic subduction zone drives frequent seismic activity. The region experiences both shallow crustal earthquakes and deeper events associated with slab subduction, with the highest hazard concentrated along the Hellenic Arc and in the Aegean Sea. Active fault systems, including strike-slip and normal faults, accommodate the rapid extension and rotation of the Aegean microplate.

Seismic swarms, defined as clusters of earthquakes without a dominant mainshock, occur periodically in Greece. Historical records maintained by SeismoSight indicate three prior swarms since 2000: one each in 2003, 2008, and 2011. These episodes typically involve low-to-moderate magnitude events at shallow depths and reflect fluid migration or stress redistribution along pre-existing faults rather than large tectonic ruptures.

Swarm S20141118.1 began at 23:05 on 17 November 2014 and concluded at 10:23 on 23 November 2014, spanning 131 hours and 17 minutes. During this interval, 103 earthquakes were recorded. The sequence initiated with two events exceeding magnitude 5.0 within the first four minutes, followed by a rapid decay in event rate and magnitude. Depths ranged predominantly between 1 km and 20 km, consistent with shallow crustal seismicity in the Aegean.

Analysis of the first 100 events reveals a classic swarm pattern: an initial energetic phase dominated by the largest shocks (magnitudes 5.4, 5.1, and 4.5), succeeded by hundreds of smaller aftershocks. Magnitudes clustered between 2.0 and 3.5 after the first day, with occasional spikes to 4.2 and 3.5. Depths remained shallow, averaging near 8 km, suggesting activity confined to the brittle upper crust. Temporal distribution showed highest productivity in the opening 24 hours, with event frequency declining steadily thereafter.

The swarm's characteristics align with known Aegean swarm behavior, where short-lived clusters rarely produce damaging ground motion yet provide valuable data on local stress fields. No damage or casualties were associated with this episode.

References

SeismoSight internal swarm catalogue (S20141118.1 parameters and historical swarm list).
Hellenic Arc tectonic framework derived from standard geological literature on Aegean plate boundary processes.